Underground utility lines may emit passive electromagnetic signals for various reasons. For example, an underground alternating current electric utility line carries its own electric signal at a given frequency that produces an electromagnetic field about the utility line. Underground cable television lines similarly carry electric signals at their own unique frequency. Other utility lines may not inherently carry an electric signal that produces an electromagnetic field but are nevertheless susceptible of having an electromagnetic field induced upon the line by the application of an electrical signal on the utility line, for example as applied by a cable locating system for the purpose of identifying the utility line's position. For example, natural gas providers typically place an electric signal on underground gas lines to provide cathodic protection against corrosion. Other utility lines such as sewer or plumbing lines are similarly susceptible to inducement of an electromagnetic field along the utility line. Any such electromagnetic field emitted by a utility line, whether generated by signals carried directly by the utility or induced on the utility by electric signals ambient to the utility, as opposed to a signal applied by a utility location system for the purpose of locating the utility, is considered a “passive” signal.
Various forms of portable equipment are known for operation above-ground to locate an underground utility via a coil antenna or, more typically, multiple antennae in differing axial orientations, tuned to detect passive signals within a predetermined frequency range. Since the underground utility line may be considered, from the perspective of a locator at a single point above the utility, to be an infinite line, the signal emitted by underground utility lines forms a generally cylindrical magnetic field coaxially about the utility. This electromagnetic field does not induce a significant signal in a coil antenna when the antenna coil axis is perpendicular to the magnetic field flux lines but will induce a peak signal in the coil antenna when the antenna coil axis is parallel or tangent to the magnetic field flux lines. The locating equipment includes an onboard processor connected to the antennas and in turn connected to a display, to indicate visually to the operator the location of the underground utility. The visual indication on the display is based on the strength and orientation of the radiated electromagnetic signals in conjunction with the operator's manipulation of the locator when receiving those signals to align the locator with the underground utility, according to the programming of the processor. An example of an above-ground cable locator is provided in U.S. Pat. No. 7,088,105, the entire disclosure of which is incorporated herein for all purposes.
Various methods may be utilized by the operator to locate the general position of, and then home in on, the underground location of the utility, e.g., carrying the locator while walking in a line believed to be perpendicular to the expected underground direction of the utility or, in cases in which the general location of the utility is not known, by walking in a grid pattern while carrying the locator, until signals are located. Depending on the locator's configuration, the operator may then manipulate the locator's position and/or orientation until signal conditions detected by the locator indicate the operator has identified an above-ground location above the utility and the utility's direction. The operator then applies a location mark or marks on the above-ground surface to indicate the detected utility line's location and direction.
Such utility location marks are not always precisely above the underground utility, for example due to factors such as the presence of a surface obstruction such as pavement, rocks or vegetation, or factors such as distortion of or inaccuracies in the radiated electromagnetic field, or human error. In instances where the operator has located such an above-ground location, but upon excavation does not uncover the utility, the operator may be unaware in which direction to continue the excavation.